1. Field of the Invention
Embodiments of the present invention relate to a recording device which forms an image on a sheet. Specifically, the embodiments of the present invention relate to a conveyance device having a suction conveyance unit that suctions and conveys various sizes of sheets of materials, which are recording media, using airflows.
2. Description of the Related Art
In a conveyance device used for an image forming device, such as a conventional inkjet printer, when an image is formed, it may be necessary to maintain a constant spacing between a recording unit and a recording medium. Thus, it is not possible to convey the recording medium while holding the recording medium. Therefore, when the recording medium is conveyed, it may be necessary that a deformation or a lifting of the recording medium is regulated. Further, during an image forming process, when a deformation of the recording medium itself occurs, for example, when stretching of the recording medium associated with adhesion of ink (hereinafter, referred to as stretching) or deformation of the recording medium associated with application of heat occurs, it may be necessary to provide a groove portion so that the deformation of the recording medium is released to the opposite side of the recording unit. As described above, the following conveyance device is known. Namely, as shown in FIG. 8, the conveyance device supports the reverse side of the recording medium on a conveyance surface in an image forming area. Further, the conveyance device includes plural suction ports for suctioning the recording medium from the reverse side, so as to prevent the recording medium from being lifted. The conveyance device conveys the recording medium while suctioning the recording medium in the image forming area.
In the above described conveyance device, when the recording medium is conveyed to a position where the recording medium covers all the suction ports 22 of orifices 14, which are arranged on the conveyance surface, since inside of each orifice 14 is sealed, a negative pressure increases and the suction force increases. In this manner, a deformation of the recording medium which exists prior to forming an image or a deformation of a recording medium caused by adhered ink when forming the image is suppressed, and the recording medium can be stably conveyed. Further, since the orifice 14 has a constant length, when a deformation, such as the stretching, occurs on the recording medium, an amount of the deformation can be released to the reverse side of the recording unit by the suction force.
However, at a position shown in FIG. 9, where the recording medium only covers suction ports 22 placed at a front tip or a rear tip of the orifice 14, the air is suctioned through suction ports 22 which are released. Thus, there is a problem that a lifting or a deformation of the recording medium is not sufficiently regulated, since the negative pressure inside the orifice 14 does not increase and the suction force does not increase. Therefore, techniques for controlling a suction force acting onto the recording medium or a suction timing to suction the recording medium in the above described conveyance device are disclosed, for example, in Patent Document 1 (Japanese Published Unexamined Publication No. S64-43430), Patent Document 2 (Japanese Published Unexamined Publication No. H09-058897), and Patent Document 3 (Japanese Registered Patent No. 3690182).
Patent Document 1 discloses a technique such that, when a sheet covers suction ports, an output of a pump is reduced so as to prevent a suction force from becoming rapidly large. However, in this configuration, the air is always suctioned through suction ports other than the suction ports conveying the sheet. In such a case, it may be necessary to adjust the suction force of the pump, so that the sheet is surely conveyed, even if the sheet does not cover all the suction ports. Thus a large pump tends to be used. Therefore, the configuration disclosed in Patent Document 1 has problems that the device becomes large, and noise and cost increase.
Patent Document 2 discloses a technique for avoiding unnecessary suctioning. In the technique, for the suction ports other than the suction ports conveying the sheet, a driving means for opening and closing a valve system and a sensor are used, so as to selectively avoid unnecessary suctioning. With this configuration, the size of the pump can be reduced. However, for example, when a surface for suctioning the sheet is large, in order to selectively avoid unnecessary suctioning at finer areas so as to prevent tips of the sheet from lifting, it may be necessary to place plural driving units for opening and closing the valve system. This leads to complication of the device and growth in size of the device. Further it can be a cause of a cost increase.
Patent Document 3 discloses a technique using a pressure adjustment valve. Namely, when a sheet covers a suction port, the pressure adjustment valve causes a suction force to be generated, and when the sheet does not cover the suction port, the pressure adjustment valve does not allow unnecessary suctioning to be performed. However, in this technique, a suction force is not obtained until the sheet completely covers an orifice (a suction port). Thus, the suction force at the tip of the sheet is always weaker than the suction force at the center of the sheet. Therefore, there is a problem that the technique does not handle the lifting of the tips of the sheet. Further, in this technique, the pressure adjustment valve is closed by fluid flow (wind). However, when the pressure adjustment valve is closed, the fluid does not flow, and the pressure adjustment valve is opened. Patent Document 3 does not disclose a technique to overcome this problem. However, as a technique to overcome this problem, a method in which the valve unit is not completely closed and the valve unit partially leaks the fluid, or a method of completely closing the pressure adjustment valve by a pressure difference between a suction side and an orifice side, the pressure adjustment valve being pinched between the suction side and the orifice side, can be considered. However, when the pressure adjustment valve is not completely closed, since not all the unnecessary suctioning is stopped, inefficiency of the suctioning remains. Further, in the method of completely closing the pressure adjustment valve by the pressure difference between the suction side and the orifice side, at an instant when the recording medium covers the orifice, the pressure difference for closing the pressure adjustment valve is not released, and the pressure adjustment valve is not opened. When the pressure inside the orifice is increased by leakage through a gap between the orifice and the recording medium, the pressure adjustment valve is opened. Therefore, the problem that the technique does not handle the lifting of the tips of the sheet remains. Further, in order to cause the pressure adjustment valve to be opened when a sheet covers a portion of an orifice, a valve may be opened and closed in response to a very small pressure difference or a very small flow difference of a fluid. Therefore, a highly precise valve system may be required, and a suction force from a suction mechanism may be very precise. Such a technique is not realistic, and it leads to problems, such as a cost increase, degradation of proccessability, and growth in size of the device.